All questions of Sequential Logic Circuits for Electrical Engineering (EE) Exam

Basic S-R Flip-Flop

The basic S-R (Set-Reset) flip-flop is a type of sequential logic circuit that can store one bit of information. It is constructed by cross-coupling two NOR (Negative-OR) gates.

Cross-Coupling of NOR Gates

To understand why the cross-coupling of NOR gates is used to construct an S-R flip-flop, let's first examine the truth table of a NOR gate:

A	B	Output
0	0	1
0	1	0
1	0	0
1	1	0

Set and Reset Inputs

In an S-R flip-flop, the two inputs are called the Set and Reset inputs. When the Set input is high (1), it sets the flip-flop to the high (1) state. Conversely, when the Reset input is high (1), it resets the flip-flop to the low (0) state.

Construction of S-R Flip-Flop

To construct an S-R flip-flop using NOR gates, we connect the output of one NOR gate to the Set input of the other NOR gate, and vice versa. This creates a feedback loop between the two gates, allowing the flip-flop to store information.

Working Principle

1. Initially, both inputs are low (0), and the outputs of both NOR gates are high (1).
2. When the Set input is activated (high), the output of the first NOR gate goes low (0), which is fed back to the Set input of the second NOR gate. This causes the output of the second NOR gate to go high (1), setting the flip-flop to the high state.
3. If the Set input returns to low (0), the flip-flop remains in the high state.
4. Similarly, when the Reset input is activated (high), the output of the second NOR gate goes low (0), which is fed back to the Reset input of the first NOR gate. This causes the output of the first NOR gate to go high (1), resetting the flip-flop to the low state.
5. If the Reset input returns to low (0), the flip-flop remains in the low state.

Summary

In summary, the cross-coupling of NOR gates allows the construction of a basic S-R flip-flop. The Set and Reset inputs control the state of the flip-flop, and the feedback loop between the gates maintains the stored information. This flip-flop is a fundamental building block in digital systems, and its behavior can be further modified and enhanced using additional logic gates.

Propagation delay is the time it takes for a signal to propagate through a logic gate or flip flop. In the context of counter design, the propagation delay of flip flops plays a significant role in determining the speed of operation, especially in asynchronous (ripple) counters.

Asynchronous counters are designed using a series of flip flops, where the output of one flip flop serves as the clock input for the next flip flop in the chain. Each flip flop introduces a certain amount of propagation delay, which is the time it takes for the output of the flip flop to stabilize after a change in its inputs.

The main reason why the propagation delay of flip flops affects the speed of operation in asynchronous counters is due to the ripple effect. When the input to the first flip flop changes, it takes some time for the output of that flip flop to stabilize. This delay in stabilization propagates through the subsequent flip flops in the chain, causing a delay in the counting sequence.

The propagation delay can lead to several issues in asynchronous counters:

1. Skew: The propagation delay of flip flops can cause a skew in the counting sequence. Skew refers to the difference in arrival times of the clock signal at different flip flops. This can result in an inconsistent and unpredictable counting sequence.

2. Glitches: The propagation delay can also cause glitches in the counter output. A glitch is a temporary and unwanted change in the output of a logic circuit. Due to the ripple effect, glitches can occur when the outputs of the flip flops are changing during the stabilization period.

3. Limited frequency of operation: Asynchronous counters have a limited operating frequency due to the cumulative effect of propagation delays. If the input clock signal has a high frequency, the propagation delays can accumulate and result in errors or unpredictable behavior in the counter.

On the other hand, synchronous counters, both up and down, do not suffer from these issues because they use a common clock signal to synchronize all flip flops simultaneously. The propagation delay of individual flip flops still exists but does not affect the counting sequence or introduce glitches, as all flip flops are updated at the same time.

In conclusion, the propagation delay of flip flops largely affects the speed of operation in asynchronous (ripple) counters due to the ripple effect, which can lead to skew, glitches, and limitations on the operating frequency.

Explanation:

The T-flipflop is a type of flipflop that toggles its output based on the input signal. It has two inputs: T (toggle) and CLK (clock) and one output Q.

Given:
Input signal frequency = 200 Hz

Working:
When a T-flipflop receives a pulse at its CLK input, it toggles its output (Q) based on the value of the T input. If T = 0, the output remains the same, and if T = 1, the output flips.

In a cascade connection of three T-flipflops, the output of one flipflop is connected to the CLK input of the next flipflop. This means that the output of the first flipflop becomes the input to the second flipflop, and so on.

Since the input signal to the first T-flipflop is a 200 Hz signal, the output of this flipflop will toggle at 200 Hz. This means that the output will change its state (from 0 to 1 or from 1 to 0) 200 times per second.

Now, the output of the first flipflop becomes the input to the second flipflop. Since the output of the first flipflop toggles at 200 Hz, the input to the second flipflop will also toggle at the same frequency.

Similarly, the output of the second flipflop becomes the input to the third flipflop. So, the input to the third flipflop will also toggle at 200 Hz.

Therefore, the final output of the three T-flipflops in cascade will also toggle at 200 Hz.

Conclusion:
The final output of the three T-flipflops in cascade is 200 Hz.

Therefore, the correct answer is option 'A' (25 Hz).

Microseconds.

Output goes lowc)Q output remains unchangedd)Q output toggles between high and low

Master-slave flip flop is a type of sequential logic circuit that can store one bit of binary information. It is commonly used in digital electronics for various applications such as memory elements, counters, and registers. This type of flip flop is also referred to as a pulse-triggered flip flop because it changes its output state in response to a specific pulse.

The master-slave flip flop consists of two D flip flops, namely the master and the slave flip flops. These two flip flops are connected in series, with the output of the master flip flop connected to the input of the slave flip flop.

The operation of the master-slave flip flop can be divided into two phases: the master phase and the slave phase.

Master Phase:
During the master phase, the master flip flop is enabled, and the input signal is allowed to propagate through the circuit. The input signal is applied to the master flip flop, and its state is stored. However, the output of the master flip flop is not directly connected to the output of the circuit.

Slave Phase:
During the slave phase, the master flip flop is disabled, and the slave flip flop is enabled. The stored state of the master flip flop is transferred to the slave flip flop, which then becomes the output of the circuit. This transfer of data occurs when a clock pulse triggers the transition from the master phase to the slave phase.

The clock pulse acts as a trigger for the master-slave flip flop. It synchronizes the operation of the flip flop and ensures that the state transfer occurs at the appropriate time.

In summary, the master-slave flip flop is referred to as a pulse-triggered flip flop because it changes its output state in response to a clock pulse. This type of flip flop is widely used in digital electronics due to its reliable and synchronized operation.

Effect of SET input in S-R latch:
When the SET input of an S-R latch is made high, it sets the latch, causing the Q output to become high.

Explanation:
- When the SET input is activated, it overrides the RESET input and forces the Q output to be high.
- This behavior is due to the internal logic of the S-R latch, where the SET input triggers the latch to store a high output.
- The Q output will remain high until a reset signal is applied to the latch.

Conclusion:
In summary, when the SET input of an S-R latch is made high, the output Q will be set to a high state, which can be represented as logic level 1.

The correct answer is option 'C', 8 states.

Explanation:
To understand why the maximum states possible for the counter formed by three T flip-flops is 8, we need to understand the operation of a T flip-flop and how it can be used to build a counter.

1. T Flip-Flop:
A T flip-flop is a type of flip-flop that toggles its output based on the input signal. It has a single input (T) and a single output (Q). The output changes state (toggles) when the input is high (1) and remains the same when the input is low (0).

2. Counter Operation:
A counter is a digital circuit that counts the number of events or occurrences of a specific signal. In this case, we are using three T flip-flops to build a counter.

- Initially, all the flip-flops are reset to 0.
- When a clock pulse is applied, the first flip-flop (FF1) toggles its output.
- The output of FF1 is connected to the input of the second flip-flop (FF2).
- When the next clock pulse is applied, FF2 toggles its output based on the input from FF1.
- The output of FF2 is connected to the input of the third flip-flop (FF3).
- When the next clock pulse is applied, FF3 toggles its output based on the input from FF2.

3. Maximum States:
To determine the maximum states possible for the counter, we need to consider the number of unique combinations of outputs from the three flip-flops.

- Each flip-flop has two possible output states: 0 or 1.
- For three flip-flops, the total number of possible combinations is 2^3 = 8.

Therefore, the maximum states possible for the counter formed by three T flip-flops will be 8 (option 'C'). These states can be represented by the binary numbers from 000 to 111.

S-R Flip-Flop as a Switch Debouncer

Switch debouncing is a process used to eliminate the noise or chatter caused by mechanical switches when they are being pressed or released. This noise can result in multiple false transitions, which can interfere with the proper functioning of digital circuits. One way to debounce a switch is by using an S-R flip-flop.

What is an S-R Flip-Flop?
A flip-flop is a sequential logic circuit that can store one bit of information. The S-R (Set-Reset) flip-flop is one of the most widely used types of flip-flops. It has two inputs, S (set) and R (reset), and two outputs, Q (output) and Q' (complement of the output).

Operation of an S-R Flip-Flop
- When the S input is HIGH and the R input is LOW, the flip-flop is set, and the Q output goes HIGH.
- When the R input is HIGH and the S input is LOW, the flip-flop is reset, and the Q output goes LOW.
- When both inputs are HIGH, the flip-flop is in an indeterminate state, which is not desirable for proper operation.

Switch Debouncing with an S-R Flip-Flop
To use an S-R flip-flop as a switch debouncer, the switch is connected to the S input, and its complement (inverse) is connected to the R input. The Q output of the flip-flop is then connected to the input of the digital circuit.

When the switch is pressed or released, it may generate multiple transitions due to mechanical bouncing. These transitions may cause the digital circuit to behave unexpectedly. However, the S-R flip-flop eliminates these unwanted transitions by ensuring that the Q output stabilizes at the final desired state.

Working Principle
- Initially, both the S and R inputs of the flip-flop are LOW.
- When the switch is pressed, the S input goes HIGH, causing the flip-flop to set and the Q output to go HIGH.
- As long as the switch remains pressed, the S input remains HIGH, but the flip-flop does not change its state due to the stable latch behavior.
- When the switch is released, the S input goes LOW, but the flip-flop maintains its state, and the Q output remains HIGH.
- Similarly, when the switch is pressed again, the R input goes HIGH, causing the flip-flop to reset and the Q output to go LOW.
- As long as the switch remains pressed, the R input remains HIGH, but the flip-flop does not change its state.
- When the switch is released again, the R input goes LOW, but the flip-flop maintains its state, and the Q output remains LOW.

Advantages of Using an S-R Flip-Flop for Switch Debouncing
- Simple and cost-effective solution for eliminating switch noise.
- Provides a stable output state unaffected by switch bouncing.
- Can be easily implemented using basic logic gates.
- Suitable for both manual and mechanical switches.

Conclusion
Using an S-R flip-flop as a switch debouncer is an effective way to remove noise and ensure the proper functioning of digital circuits. By connecting the switch to the S input and its complement to the R input, the flip-flop provides a stable output state

SR flip-flop and JK flip-flop are two commonly used sequential logic circuits in digital electronics. They both store and manipulate binary information, but they have some functional differences. In this case, the correct answer is option 'C', which states that the JK flip-flop accepts both inputs 1.

SR Flip-Flop:
- An SR flip-flop (Set-Reset flip-flop) is a basic latch circuit that can store one bit of information.
- It has two inputs: Set (S) and Reset (R).
- When S=0 and R=0, the flip-flop holds its previous state.
- When S=1 and R=0, the flip-flop is set, meaning the output Q becomes 1.
- When S=0 and R=1, the flip-flop is reset, meaning the output Q becomes 0.
- When S=1 and R=1, it is considered an invalid state, and the output is unpredictable.

JK Flip-Flop:
- A JK flip-flop is an improved version of the SR flip-flop that overcomes the invalid state problem.
- It has three inputs: J (Set), K (Reset), and Clock (C).
- When J=0 and K=0, the flip-flop holds its previous state.
- When J=1 and K=0, the flip-flop is set, meaning the output Q becomes 1.
- When J=0 and K=1, the flip-flop is reset, meaning the output Q becomes 0.
- When J=1 and K=1, the flip-flop toggles its state. If the previous state was 0, Q becomes 1, and if the previous state was 1, Q becomes 0.

Functional Difference: JK flip-flop accepts both inputs 1
- The functional difference between SR flip-flop and JK flip-flop is that the JK flip-flop accepts both inputs 1, whereas the SR flip-flop does not.
- When both J and K inputs are set to 1 in a JK flip-flop, it toggles its state. This allows for more flexibility and control in circuit design.
- In contrast, if both S and R inputs are set to 1 in an SR flip-flop, it enters an invalid state, resulting in unpredictable behavior.
- By allowing both inputs 1, the JK flip-flop can be used to implement more complex logic functions and sequential circuits.
- This makes the JK flip-flop more versatile and widely used compared to the SR flip-flop.

Overall, the ability of the JK flip-flop to accept both inputs 1 gives it an advantage over the SR flip-flop in terms of functionality and flexibility in digital circuit design.

Basic Latch

A basic latch is a fundamental building block in digital electronics. It is a simple circuit that can store a single bit of information. The basic latch consists of two inverters connected in a feedback loop.

Working Principle

The latch operates based on the principle of positive feedback. When an input signal is applied to one of the inverters, it produces an inverted output. This inverted output is fed back to the input of the other inverter, causing a continuous loop.

Feedback Loop

The feedback loop in the latch ensures that the output state remains stable even after the input signal is removed. It allows the latch to "remember" the previous state until a new input signal is received.

Set and Reset

The basic latch has two inputs: set and reset. The set input is used to store a '1' or logic high state, while the reset input is used to store a '0' or logic low state.

- When the set input is activated, it drives one of the inverters to produce a logic high output. This high output propagates through the feedback loop, reinforcing the high state.
- Similarly, when the reset input is activated, it drives the other inverter to produce a logic low output. This low output propagates through the feedback loop, reinforcing the low state.

State Retention

Once the latch is set or reset, it retains its state even if the input signals are removed. This is due to the positive feedback loop that maintains the output state until a new input signal is applied.

Level-Sensitive Latch

The basic latch described above is known as a level-sensitive latch. It is sensitive to the levels of the input signals and changes its state accordingly. It is commonly used in sequential logic circuits, such as flip-flops.

Conclusion

In summary, the basic latch consists of two inverters connected in a feedback loop. It operates based on positive feedback and can store a single bit of information. The latch has set and reset inputs, which allow it to store either a logic high or logic low state. The positive feedback loop ensures that the latch retains its state until a new input signal is received.

Synchronous Control Inputs in a T Flip Flop

The synchronous control inputs in a T flip flop are used to control the state of the flip flop based on the input signals. In a T flip flop, the state of the flip flop is toggled or inverted based on the input signal applied to the T input.

T Flip Flop

A T flip flop is a type of flip flop that has a single input called the T input. The T input is connected to the clock signal, and the state of the flip flop changes on each rising or falling edge of the clock signal, depending on the value of the T input. The output of the flip flop is the complement of its previous state.

Synchronous Control Inputs

The synchronous control inputs in a flip flop are the inputs that are synchronized with the clock signal and affect the state of the flip flop on each clock cycle. In the case of a T flip flop, the synchronous control input is the T input.

Options

a) T
b) 0
c) Clock
d) 1

Explanation

In the given options, the only option that represents the synchronous control input in a T flip flop is option 'a' - T. The T input is used to toggle or invert the state of the flip flop on each clock cycle.

Option 'b' - 0 is not a synchronous control input. It represents a logic low or zero input, but it does not control the state of the flip flop.

Option 'c' - Clock represents the clock signal itself, which is not a synchronous control input. The clock signal is used to synchronize the operation of the flip flop, but it does not directly control the state of the flip flop.

Option 'd' - 1 is also not a synchronous control input. It represents a logic high or one input, but it does not control the state of the flip flop.

Therefore, the correct option representing the synchronous control input in a T flip flop is option 'a' - T.

C) They are externally reset or set